1. Field of the Invention
The present invention relates to an optical element and an optical system using the same and, more particularly, to an optical system for a video camera, still video camera viewfinder, or a copying machine using an optical element integrating a plurality of reflection surfaces with curvatures.
2. Related Background Art
Conventionally, a variety of optical systems using the reflection surfaces of concave mirrors or convex mirrors have been proposed.
For example, U.S. Pat. No. 4,775,217 or Japanese Patent Application Laid-Open No. 2-297516 discloses an optical prism whose optical block has a reflection surface with a curvature.
U.S. Pat. No. 4,775,217 is associated with the arrangement of the eyepiece of an observation optical system. FIG. 11 shows this arrangement.
In the observation optical system shown in FIG. 11, display light 215 from an information display 211 is incident from an incident surface 218, is reflected to the object side by a total reflection surface 212, and reaches a concave surface 213 having a curvature.
The display light 215 that is output from the information display 211 as divergent light is converted into almost collimated light by the power of the concave surface 213 and enters a pupil 214 of the observer through the total reflection surface 212, so the observer recognizes the image displayed on the information display 211.
In this prior art, the object image can also be recognized simultaneously with observation of the displayed image.
Object light 216 is incident on a surface 217 nearly parallel to the total reflection surface 212 and reaches the concave surface 213. For example, a semi-transparent film is deposited on the concave surface 213. The object light 216 half-transmitted through the concave surface 213 passes through the total reflection surface 212 and enters the pupil 214 of the observer. Hence, the observer can observe the object light 216 and the display light 215 in a superposed state.
In a non-coaxial optical system, when asymmetrical aspherical surfaces are formed as constituent surfaces on the basis of the idea of a reference axis (to be described later), a compact observation optical system whose aberration is sufficiently corrected can be constructed. Japanese Patent Application Laid-Open No. 9-5650 discloses a method of designing the optical system. Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372 disclose examples of design.
Such a non-coaxial optical system is called an off-axial optical system (An off-axial optical system is defined as an optical system including surfaces (off-axial optical surfaces) whose surface normals at the intersections between the surfaces and a reference axis are not present on the reference axis which is along a light beam passing through the image center and the pupil center. At this time, the reference axis bends).
In this off-axial optical system, generally, the constituent surfaces are non-coaxial, and the reflection surfaces do not generate an eclipse. For this reason, an optical system using reflection surfaces can be easily constructed. In addition, an integrated optical system can be easily constructed by integrally molding the constituent surfaces. With this method, the optical path can be relatively freely guided.
Hence, a compact reflection optical element with a high space efficiency and free shape can be formed.
However, in the integrally molded optical block of the off-axial optical system, when the number of reflection surfaces is increased for the purpose of, e.g., aberration correction of the optical block, influences of surface shape errors or surface distortion as manufacturing errors of the reflection surfaces accumulate. The error amount allowable in each reflection surface becomes smaller and stricter as the number of reflection surfaces increases. For this reason, the surface shape of each reflection surface must be accurately guaranteed.
An optical system with a small image size, which is disclosed in, e.g., Japanese Patent Application Laid-Open No. 8-292371 or 8-292372, has large curvatures, and the required accuracy against surface errors or surface distortion is high.
This also applies to the observation optical system disclosed in U.S. Pat. No. 4,775,217 or Japanese Patent Application Laid-Open No. 2-297516 when a compact high-performance optical system is constructed.
The characteristic features of the reflection optical elements disclosed in Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372 will be described next.
FIG. 12 shows an embodiment disclosed in Japanese Patent Application Laid-Open No. 8-292371. This optical system has an intermediate imaging plane N1 and a pupil N2 of the optical system. The intermediate imaging plane is formed near a second reflection surface R4 counted from an incident surface R2 along the optical path, i.e., one of reflection surfaces having curvatures.
The pupil is formed near a second reflection surface R6 reversely counted from the exit surface along the optical path, i.e., one of the reflection surfaces having curvatures. If a first reflection surface R3 having a curvature, which is counted from the incident surface R2 along the optical path, has a convergence function, the intermediate imaging plane N1 readily forms near the above-described reflection surface R4. If a final reflection surface R7 having a curvature, which is counted from the incident surface along the optical path, has a convergence function, the pupil N2 readily forms near the above-described reflection surface R6.
These surfaces are sensitive to distortion and spherical aberration, so the surface shapes must be accurately guaranteed.
To form an optical block having a plurality of reflection surfaces, molding using a mold is widely used because of the recent requirement for simplicity. When the mold is larger than the optical effective portion to some extent, the influence of the surface distortion near the reflection surfaces on the optical effective portion becomes small.
A large mold is also advantageous in guaranteeing the positional accuracy of each reflection surface. In a process using a synthetic resin, changes in dimensions due to shrinkage in molding or the use environment must be taken into consideration because the thermal expansion coefficient of the synthetic resin is larger than that of an inorganic material by one order of magnitude. In association with the optical characteristics, not only the molding accuracy but also molding shrinkage and molecular orientation need to be taken into consideration.
Molding shrinkage influences the dimension accuracy of the entire molded body. Local shrinkage in cooling appears as residual distortion or deformation. Generally, when a molding material hardens in a mold, shrinking stress remains because the material cannot freely shrink. When a molded body formed from a soft material is released from a mold, such stress is released to warp the molded body. For a hard material such as polystyrene, polymethyl methacrylate, or polycarbonate, stress is not released, and a molded body maintains its shape with residual stress.
This stress is called internal stress. When the molded body comes into contact with, e.g., a solvent, a crack readily forms. The molded body may spontaneously break during use.
In consideration of this problem, Japanese Patent Application Laid-Open No. 8-122505 discloses an examination in which when a plurality of optical components are to be integrally formed as one optical member, contact surfaces at a joint portion are formed into appropriate shapes, and two surfaces adjacent to each other are smoothly joined at the boundary. This decreases residual stress on the optical member in the molding process to reduce manufacturing errors.
However, not all optical members can always be smoothly joined. When a design is made to smoothly join an optical member, the optical performance cannot be maintained.
FIG. 8 is a perspective view showing the surface shapes of a reflection optical element disclosed in Japanese Patent Application Laid-Open No. 8-292371, and the incident states of an incident light beam on the reflection surfaces. A cluster of symbols “+” represents a light beam incident on each optical surface.
FIG. 9 is a perspective view showing only the surface shapes of a reflection optical element of another embodiment of this prior art.
Referring to FIG. 8, the optical element has refraction surfaces R1 and R7, reflection surfaces R2 to R6 forming two reflection surface groups opposing each other, and an image sensing element Si such as a CCD. A light beam from an object is incident from the incident surface R1, is repeatedly reflected by the reflection surfaces R2 to R6, exits from the exit surface R7, and forms an image on the image sensing element Si (image sensing surface). As shown in FIG. 8, the light is incident on the reflection surfaces in various states.
Referring to FIG. 9, the optical element has refraction surfaces R1 and R8, a reflection surface R2 which does not oppose two reflection surface groups opposing each other, reflection surfaces R3 to R7 forming two reflection surface groups opposing each other, and an image sensing element Si such as a CCD.
A light beam from an object is incident from the incident surface R1. The direction of light is changed by the reflection surface R2. The light beam is repeatedly reflected by the reflection surfaces R3 to R7, exits from the exit surface R8, and forms an image on the image sensing element Si (image sensing surface).
In the example shown in FIG. 8, the curvature of the reflection surface R3 is large. In FIG. 9, the curvature of the reflection surface R6 is large. The curvatures of surfaces adjacent to each other, i.e., the surfaces R1, R3, R5, and R7 in FIG. 8 or the surfaces R4, R6, and R8 in FIG. 9 are largely different. For this reason, it is hard to smoothly join these surfaces.
As is apparent from FIGS. 8 and 9, when a surface with a large curvature is simply extended to side surfaces while maintaining the surface shape of the optical surface, or when cross sections of adjacent surfaces with largely different curvatures are simply joined as shown in FIGS. 10A, 10B, and 10C, the resultant optical element has sharp ridge portions C1 on the side surfaces or a step at a joint portion C2 between the optical surfaces.
Generally, when reflection surfaces are larger than the optical effective portion to some extent, it is advantageous to guarantee the surface accuracy of or positional accuracy of each reflection surface. In this case, an accurate surface shape can be guaranteed by ensuring the placing of an optical reflection surface to a portion near the optical effective portion and compensating for other portions using shapes different from the shape of the optical reflection surface.
As shown in FIGS. 8 and 9, a surface having a large curvature and small optical effective portion is often convex facing the inside of the device.
When surfaces before and after such a surface have a convergence function, the optical effective portion of this surface inevitably becomes small.
As described above, these surfaces are sensitive to distortion and spherical aberration, so the surface shapes must be accurately guaranteed. Generally, a molding auxiliary portion such as a draft or an ejection portion formed from an ejector pin is prepared at a predetermined position of the reflection optical element, thereby suppressing manufacturing errors. To suppress manufacturing errors and guarantee the optical performance, the molding auxiliary portion is preferably formed near a reflection surface.
An optical element represented by Japanese Patent Application Laid-Open No. 8-292371 aims at forming a compact and free shape. For this reason, the degree of freedom in forming a draft or an ejection portion by an ejector pin is low, like conventional optical elements.
Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372 disclose a means for obtaining the surface shape of the optical effective portion. Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372 do not disclose any specific method of forming shapes other than the optical surface.